Ni-Ti alloys with a nickel content comprised between 40 and 52 atomic % pertain to the category of thermoelastic materials (also known in the field as Nitinol, Shape Memory Alloys, “smart” materials, etc). According to the finishing process these alloys undergo (e.g., training, shape setting, etc), they may exhibit a shape memory effect or a superelastic behavior. Details of suitable processes and characteristics of these alloys are widely known in the art and may be found, for example, in C. M. Wayman, “Shape Memory Alloys” MRS Bulletin, April 1993, 49-56, M. Nishida et al., “Precipitation Processes in Near-Equiatimic TiNi Shape Memory Alloys”, Metallurgical Transactions A, Vol 17A, September, 1986, 1505-1515, and H. Hosoda et al., “Martensitic transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions”, Intermetallics, 6(1998), 291-301, all of which are herein incorporated by reference in their entirety.
These alloys are employed in a variety of applications. By way of example and not of limitation, in industrial applications, shape memory wires are used in actuators as a replacement for small motors. Further applications for such thermoelastic materials include the medical field, where they are used for stents, guidewires, orthopedic devices, surgical tools, orthodontic devices, eyeglass frames, thermal and electrical actuators, etc.
Independently from the final shape of the Ni-Ti thermoelastic device, which can, for example, be wire-, tube-, sheet- or bar-based, the manufacturing process includes a cutting phase from a longer metallic piece, obtained from a semi-finished product resulting from an alloy melting process as described, for example, in U.S. Pat. No. 8,152,941, assigned to the same assignee of the present application and incorporated herein by reference in its entirety. The most common forms for the semi-finished products are long tubes, wires, rods, bars, sheets.
The behavior of these Ni-Ti alloys is strongly dependent on their composition. The presence of one or more additional elements may result in new properties and/or significantly alter the characteristics and behavior of the alloy.
A way to improve the characteristics and properties of semi-finished product made with Ni-Ti alloys with the addition of controlled amounts of other elements has been addressed in the aforementioned U.S. Pat. No. 8,152,941. The purity of Ni-Ti alloys with respect to gaseous content has been addressed in international patent application number WO 2005/049876, also incorporated herein by reference in its entirety.
There is also a need to improve the characteristics of the Ni-Ti base alloy such as the alloy described in the ASTM standard F 2063, with particular reference to the alloy chemical composition as stipulated in Table 1 of F2063. These improvements lead to alloys with better properties and consequently to final devices with improved characteristics, especially in terms of fatigue resistance. As an example, improvements over the ASTM standard for a different type of alloy, Co-Cr-Mo are described in U.S. Pat. No. 8,048,369, also incorporated herein by reference in its entirety.